Question

If I have objects a and b and both reference object obj, what happens when I Pickle and then restore the objects? Will the pickled data ‘know’ that a and b both referenced the same object and restore everything accordingly, or will the two get two different — and initially equal — objects?

Answer 1

Yes, shared objects will only get serialized once (the pickle protocol can even handle circular references).

From the documentation:

The pickle module keeps track of the objects it has already
serialized, so that later references to the same object won’t be
serialized again. marshal doesn’t do this.

This has implications both for recursive objects and object sharing.
Recursive objects are objects that contain references to themselves.
These are not handled by marshal, and in fact, attempting to marshal
recursive objects will crash your Python interpreter. Object sharing
happens when there are multiple references to the same object in
different places in the object hierarchy being serialized. pickle
stores such objects only once, and ensures that all other references
point to the master copy. Shared objects remain shared, which can be
very important for mutable objects.